The contract is a record number in total dollars paid for a free-agent amateur. Castillo was en route to Boston on Friday, according to ESPN's Pedro Gomez, and was expected to undergo a physical on Saturday.
Red Sox manager John Farrell has seen and read the scouting reports on the 27-year-old outfielder.
"Above-average speed. Can play center field or right field. What kind of power? What kind of average? Obviously our scouts liked him enough, if the reports are true, that's a significant investment. It's an exciting, athletic player by all accounts," Farrell said.
Farrell added that he is aware of the reports of the Castillo signing, but there are still administrative things that Castillo must go through before anything is announced officially.
The contract begins this season, the source said, which makes the average annual value of the deal (for luxury-tax purposes) $10.36 million, a tick below the $11.3 million AAV of the six-year, $68 million deal that Cuban slugger Jose Abreu signed with the Chicago White Sox last winter. Castillo's deal runs through the 2020 season.
The Red Sox, who last winter were outbid for Abreu, won what became a furious bidding war for Castillo, 27, whose workout last month in Miami was attended by 28 major league teams. Castillo subsequently conducted private workouts for a number of teams, including the Red Sox, who in the end, according to multiple sources, held off a strong bid from the Detroit Tigers.
In coming to terms with Castillo, whom the Red Sox project as a center or right fielder, Boston has completed a remarkable transformation of an outfield that had ranked as one of the weakest offensively in club history.
The Red Sox executed trades at the deadline for outfielders Yoenis Cespedes (Oakland) and Allen Craig (St. Louis), and with holdovers Daniel Nava and Shane Victorino expected to be ready for spring training after undergoing back surgery, Boston now has an outfield crowded with experienced veterans.
The Red Sox spent much of the season with rookie Jackie Bradley Jr. playing center field and converted two other rookie infielders, Brock Holt and Mookie Betts, into outfielders. Bradley, a superb defender who hit just .216, was sent down to the minors this week, with Betts recalled to take his place. Holt, who has played every position in the infield and outfield, has served as the team's leadoff hitter since mid-May.
The flurry of activity, which cost the Red Sox pitchers Jon Lester and John Lackey in the Cespedes and Craig deals, comes less than two years after the Red Sox hit the reset button in spectacular fashion, unloading stars Adrian Gonzalez, Carl Crawford and Josh Beckett in an August deal to the Los Angeles Dodgers that freed up nearly $260 million in salary and netted the Red Sox two promising young pitchers in Rubby De La Rosa, who is currently in the Red Sox's rotation, and Allen Webster, who is in Triple-A Pawtucket and has already had multiple big league call-ups.
According to one major league talent evaluator, Castillo's power and bat tools were not as evident as they were with Abreu, but he also brings plus speed to the table and average-to-above-average defense.
Initially, according to a source with direct knowledge of the negotiations, the Red Sox did not expect the bidding for Castillo to approach Abreu's salary level, but fueled by strong competition from other clubs that included the Tigers and reportedly the Yankees, Phillies, Mariners and Giants, the dollars surpassed those of Abreu, whose 32 home runs are tied with Miami's Giancarlo Stanton for second in the majors, one behind Baltimore's Nelson Cruz.
Castillo, who is listed at 5-foot-9 and 185 pounds (205 pounds in some reports), played five seasons for Ciego de Avila in Cuba's top league, Serie Nacional, and was regarded as one of the top base stealers in Cuba. He ultimately inherited the job as the national team's center fielder after the defections of Cespedes and Leonys Martin, now with the Texas Rangers.
His coming-out performance came in the World Cup in Panama in October 2011, when he batted .512 (21-for-41) with eight extra-base hits in 10 games, including two home runs.
When Castillo does make his major league debut, Farrell said, despite the outfielder's lack of experience in big leagues, it would not be complicated working the newest Red Sox player into the lineup this season.
"We're going to have a pretty good read on what his capabilities are based on where he's played in the past," Farrell said. "There's probably a growing level of comfort with these types of situations as the next Cespedes, or Abreu, or Yunel Escobar, those types of players start to pave the way for future players coming over, so you get a little bit more of a known commodity with a guy who has never played in the major leagues before. You're not always going to project to the exact number, but you're going to get a better read and feel on what they might be able to do."
Castillo was notably absent from the team that represented Cuba in the 2013 World Baseball Classic. According to Baseball America, before he defected last December, Castillo was suspended from the national team for a "violation of the code of ethics of revolutionary baseball," an indication that he had tried to defect before.
Cespedes told WEEI.com earlier this month that Castillo was "very comparable to [Yasiel] Puig. Obviously a different height and size, but very similar qualities." ESPN.com's Jayson Stark spoke to another talent evaluator who compared Castillo to Pittsburgh's Andrew McCutchen.
Castillo represents Boston's second major signing of a Cuban defector in recent years. The Red Sox also signed shortstop Jose Iglesias, who last July went to the Tigers in a three-way deal that netted the Red Sox pitcher Jake Peavy.
Because Castillo is older than 23 and played five seasons professionally in Cuba, he is exempt from international signing-bonus pools. Castillo is represented by Jay Z's Roc Nation Sports.